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Exam Grondmechanica.

Bachelor 2nd year, CiT/CTW Module/course code: 2015-201400144-1A: Veiligheid en Risico's in Delta's - Grondmechanica Date: 20th October 2016 Time: 13.45-15.45 (+25% for students who may use extra time) Module-coördinator: Maarten Krol Instructor: Vanessa Magnanimo Type of test: Exercises, Open questions. Allowed aids: Scientific calculator, formula sheet, English-Dutch dictionary. Attachments: Formula sheet. ------------------------------------------- Exercise 1 (30 points) A flownet under a dam is sketched in the Figure.

a) How many flow channels are present? b) How many equipotential drops are present? c) Identify the equipotentials and the flow lines (draw them with different

colors/styles on the test paper or on your paper). d) Determine the total flowrate under the dam, if the permeability of the soil is

k=2x10-7 cm/s. e) Calculate and plot the distributions of uplift pressure on the base of the dam. f) Describe what “equipotential lines” are and “flow lines” are.

Name:______________________  St.  nr:_______________________  

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Exercise 2 (30 points) A consolidation test on a saturated clay gave the following results: Cc=0.2 and Ce=0.04, and OCR=4.5. The existing vertical effective stress in the field is 120 kPa. A building foundation will increase the vertical total stress at the center of the clay by 80 kPa. The thickness of the clay layer is 1.5 m. The water content is 36% and the specific gravity Gs=2,65.

a) Sketch the graph of void-ratio against effective-vertical-stress (natural logarithm) and briefly describe its parts.

b) Locate the initial and final states of the soil in your plot at a). c) Calculate the consolidation settlement. d) What would be the difference in settlement if OCR was 1.5 instead 4.5?

Exercise 3 (40 points) A soil profile behind a retaining wall is made of two layers. Height of both layers is 6m while the water table is at 1.5m from the surface. Characteristics of Layer1 (sand) are c’=0, φ’=35°, γsat=19kN/m3, γd=18kN/m3. For Layer2 (clay) c’=10kPa, φ’=28° and γsat=20kN/m3.

a) Calculate the effective vertical stress, horizontal active stress and pore pressure behind on the wall in A, B, C, D and sketch the profiles.

Given that point D is in the active state as in a), consider the effective stresses. b) Draw the soil element (small square) representative of point D. c) Draw the Mohr’s circle representative of point D and the yield (failure) locus. d) Determine the maximum shear stress in point D. Calculate the corresponding

plane (orientation) and draw it on the circle and on the soil element. e) Does failure in D happens along the plane calculated in d)? Comment on your

answer.

Hw=1.5m

Layer 1

Layer 2

H=6m

base

w.t.

H=6m

D

A

B

C

Name:______________________  St.  nr:_______________________  

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Exercise 3 (40 points)

The section through a sheet piling is shown in Figure ??? The water flows only in the silt layer, while water in the sand is at rest. Consider the silt to be saturated both below and above the water table. Focus first on the case (a) when the excavation is dry (water table at the sand level).

1) Evaluate the pore pressure in front and behind the piling. Draw the pore pressure profiles.

2) Evaluate the hydraulic gradient in the silt layer. Then consider the case (b) when the water table is at level A, and evaluate again:

3) the pore pressure in front and behind the piling and draw the pore pressure profiles;

4) the hydraulic gradient in the silt layer. 5) Comment on the differences between the two cases and explain the physical

reasoning behind them.

Name:______________________  St.  nr:_______________________  

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Exercise 4 (30 points) A bridge pier is to be constructed in a riverbed by constructing a cofferdam, as shown in the figure. A cofferdam is a temporary enclosure consisting of long, slender elements of steel, concrete or timber members to support the sides of the enclosure. After construction of the cofferdam, the water within it will be pumped out. Given the flow net in the figure:

a) Calculate the flow rate of a pump required to keep the water level at the base of excavation.

b) Calculate the porewater pressures at A and B. Figure pagg. 119-120 Budu.

Exercise 5 (30 points) The flownet at a site of a reservoir is shown in the figure.

a) How many flow channels are present? b) How many equipotential drops are present? c) What is the total head loss? d) Calculate the head loss between each equipotential line. e) Calculate the average flow rate if keq=4x10-6 cm/s. f) Calculate the porewater pressure at A and B located on opposite sites of the

sheetpiling retaining wall. g) What woul happen to the wall if the critical hydraulic gradient at the bottom of it

(C) is exceeded.